A light emitting diode (LED), whose efficiency has been improved in recent years, has been widely used in a backlight of a display device and lighting equipment as a light source capable of saving more power than a light bulb or a fluorescent. Various light-emitting devices including light-emitting elements such as the LEDs have been proposed so far. In particular, a light-emitting device including a light-emitting element which is provided on a substrate and is sealed by a resin containing fluorescent material (fluorescent particles) is known.
In such a light-emitting device, short wavelength light emitted from the light-emitting element is excited by a fluorescent material, to thereby cause occurrence of secondary light. Then, the light from the light-emitting element and light from the fluorescent material are combined together, and hence the aforementioned light-emitting device can emit light of predetermined chromaticity. For example, the light-emitting device can emit white light by combining a blue light-emitting element and a yellow fluorescent material. Specifically, blue light emitted from the blue light-emitting element is converted into yellow light by the yellow fluorescent material, and then the yellow light thus converted and blue light are combined together, to thereby emit white light.
As a method for forming a sealing resin, a compression molding using a mold can be employed (see, e.g., Patent Literature 1), for example. However, during the compression molding, viscosity of a liquid resin containing fluorescent material is often reduced when a temperature is raised in order to cure the liquid resin containing fluorescent material. When the viscosity of the liquid resin is reduced, the fluorescent material in the liquid resin may be sedimented by gravity. Consequently, the fluorescent material may gather into a bottom of a concave of a lower mold, in other words, may gather into a top of the sealing resin of a finished light-emitting device. Gathering of the fluorescent material into the top of the sealing resin means that the fluorescent material is away from the light-emitting element, resulting in reduction in wavelength conversion efficiency of the fluorescent material. Therefore, a large amount of fluorescent material needs to be added, but this is not preferable in terms of cost.
Further, light emitted upwardly with respect to the light-emitting element passes through the fluorescent material, so that an emission wavelength spectrum is increased. Meanwhile, light emitted laterally with respect to the light-emitting element does not pass through the fluorescent material, so that an emission wavelength spectrum of the light emitted laterally is similar to an emission wavelength spectrum of the light-emitting element. Because of this, the light-emitting device sometimes has different emission wavelength spectra, i.e., different colors depending on the directions along which light is emitted.
For this reason, in the case of forming the sealing resin by the compression molding, a resultant sealing resin is sometimes inappropriate for the light-emitting device. The fluorescent material would not be sedimented if the mold is rotated so that the gravity is applied to the resin on the side of the light-emitting element and substrate. However, this causes a lot of problems, for example, complication of a molding machine.
In view of this, in order to (i) solve the problem of the sedimentation of the fluorescent material and to (ii) reduce the cost as much as possible in response to a demand for reduction in the total cost, a sealing method without using the compression molding, which uses the mold, has been also proposed. The sealing method without using the compression molding is as follows: a dam sheet (retaining member) having a through hole capable of accommodating a light-emitting element is attached to a surface of a substrate on which surface the light-emitting element is mounted, and then a liquid resin containing fluorescent material is injected into the through hole and thermally cured.
According to this sealing method, the sealing resin can be formed by retaining the liquid resin containing fluorescent material within the through hole with use of the dam sheet. Further, the fluorescent material is sedimented on the side of a light-emitting element even if the fluorescent material in the resin is sedimented by the gravity during the thermal curing. Consequently, the fluorescent material covers the light-emitting element, with the result that change in the emission wavelength spectrum, in other words, change in color caused by a direction is prevented.
However, in the aforementioned sealing method, the dam sheet is thrown away every time when sealing is finished. Therefore, this sealing method disadvantageously highly costs. In view of this, such a method has been proposed that uses a water-repellent film instead of the dam sheet.
For example, a method for forming a sealing layer is disclosed in Patent Literature 2 as follows: on a planar substrate having a plurality of light-emitting elements, ring-shaped frames made of a water-repellent liquid resin (for example, fluororesin) are formed so that the ring-shaped frames surround an area in which the sealing layer is formed; then, a liquid transparent resin (for example, silicone resin or epoxy resin) is injected into the ring-shaped frames and is thermally cured. The ring-shaped frames as the water-repellent films prevent the liquid resin from extending beyond the ring-shaped frames. Ultimately, the liquid resins having the same thickness can be held within the ring-shaped frames, and the sealing layer having a target dimension can be obtained.
Meanwhile, a greater improvement in luminous efficacy has been demanded in recent years. In order to meet the demand, there has been proposed a light-emitting device having a dual seal structure in which a resin containing fluorescent material for sealing a light-emitting element on a substrate is covered with a transparent resin in order to improve a luminous efficacy. In order to manufacture the light-emitting device having the aforementioned structure with use of the water-repellent film, the resin containing fluorescent material is formed so as to seal the light-emitting element with use of the water-repellent film as described above, and then the transparent resin is formed so as to cover the resin containing fluorescent material.